linux-kernel - Re: [PATCH] x86/alternatives: Fix optimize

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Message-ID: <CA+55aFw_KWncsY0x=_BnEKkZz3dFUm921ppu0HbGk8e2sBwiAA@mail.gmail.com>
Date:   Wed, 10 Jan 2018 12:49:37 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

On Wed, Jan 10, 2018 at 12:33 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Use gdb on /proc/kcore and disassemble self to see the _real_ code.

Yes, because that works so well when you get these random reports of
crazy behavior that simply doesn't ever trigger on any machine you
have.

> But yes, tricky stuff. Not arguing we need relocations in alternatives,
> just saying debugging them (and static keys and others) is great fun.

Yes. It's definitely a good challenge, and can be _very_ satisfying
when you then finally figure it out.

I'm not sure it's worth the pain before that point, though.

So I think we should strive to make our alternatives code just catch
some of the more subtle errors early instead.

           Linus